1. Field of the Invention
This invention relates to an exhaust cleaning-up device for an internal combustion engine for a vehicle, and particularly to a technique for improving the capacity to remove HC (carbon hydride) emitted when the internal combustion engine is cold.
2. Description of the Related Art
Generally, in an exhaust system of an engine (internal combustion engine), there is provided an exhaust cleaning-up device for removing harmful substances (HC, CO, NOx, etc.) from exhaust using, for example a three-way catalyst. The three-way catalyst, however, cannot fully act for this removal until it reaches its activation temperature. Even when the three-way catalyst is arranged near the engine body so that it becomes activated early, there is still a problem that HC which is emitted especially in large amounts when the engine is cold-started and cannot be removed satisfactorily.
In order to solve this problem, there has been proposed an HC absorption catalytic device in which an exhaust gas cleaning-up catalyst layer is provided on an HC absorbent layer effective for absorbing HC so that HC is absorbed into the HC absorbent layer and that HC desorbed when the HC absorbent layer reaches a certain temperature is removed by oxidation with oxygen trapped in the exhaust gas cleaning-up catalyst layer.
There is, however, a problem that while the desorption of HC from the HC absorbent starts at low temperatures about 100° C. to 150° C., the exhaust gas cleaning-up catalyst becomes activated generally at-high temperatures about 250° C. to 350° C., so that until the exhaust gas cleaning-up catalyst becomes activated, HC desorbed is not removed but discharged.
Considering problems like this, there has been developed a device arranged such that when the exhaust gas cleaning-up catalyst in the HC absorption catalytic device is at low temperature, so that, for example the amount of oxygen trapped in the exhaust gas cleaning-up catalyst layer is small and the oxygen-holding state of the exhaust gas cleaning-up catalyst has not reached a specified satisfactory level yet, the amount of oxygen contained in the exhaust is increased by setting the engine air/fuel ratio to a lean air-fuel ratio or stopping fuel supply to the engine (fuel cut) (see Japanese Unexamined Patent Publication No. 2002-38927).
Meanwhile, in recent years, an increasing number of vehicles have an automatic transmission (A/T) coupled to an output shaft of the engine. Normally, the A/T includes a hydraulic coupling that can be switched between a direct connected state and a non-direct connected state by means of a lock-up clutch (direct-connecting clutch). In the case of the A/T having such lock-up clutch, putting the lock-up clutch into a connecting state is normally prohibited while the A/T is cold. The reason is that while the A/T is cold, the frictional property of oil (such as ATF) in the transmission is unstable, so that putting the lock-up clutch into the connecting state makes variable speed control unstable and causes oscillation of the vehicle body, etc. and therefore causes deterioration in feeling.
There is, however, another problem. The period in which the A/T is cold approximately corresponds to the period in which the engine is cold-started and still cold. Thus, when the engine is cold-started, the lock-up clutch of the A/T is in the non-connecting state and the hydraulic coupling by itself cannot transmit power of the wheels to the engine. Hence, if the fuel cut is performed when the engine is cold-started, the engine revolving speed reduces drastically simultaneously with the stop of fuel supply and reaches a fuel-return revolving speed in a moment, so that fuel injection resumes. Thus, the vehicle provided with an A/T having a hydraulic coupling has a problem that when the engine is cold-started, supply of oxygen to the exhaust system by fuel cut is virtually unperformable, so that HC desorbed from the HC absorbent cannot be oxidized satisfactorily.
There is also a problem that when the engine is instantly subjected to a fuel return due to a drastic reduction in engine revolving speed caused by the fuel cut as mentioned above, hunting of the engine output and engine revolving speed occurring, so that the vehicle drivability deteriorates.
Further, it is not desirable to perform fuel cut also when the vehicle is not decelerating. Specifically, if the fuel cut is performed when the vehicle is not decelerating, sufficient engine output cannot be obtained in the period in which fuel supply is stopped, so that the vehicle drivability deteriorates.
Further, when the engine air/fuel ratio is set to a lean air/fuel ratio, a trade-off comes into question. Specifically, while HC desorbed from the HC absorbent is relatively satisfactorily removed by oxidation, lean combustion tends to cause emission of a large amount of NOx.